Prevent NPE in case no provider wiring is available

In some case when a provider wiring is not available there can be a NPE
when setting the PDE classpath, this now checks if a provider wiring is
available before access the provider.

Author: laeubi

ui/org.eclipse.pde.core/src/org/eclipse/pde/internal/core/ClasspathUtilCore.java

@@ -128,7 +128,7 @@ private static Stream<IClasspathEntry> getRequiredByDescription(BundleDescriptio
 		}
 
 		List<BundleWire> wires = wiring.getRequiredWires(PackageNamespace.PACKAGE_NAMESPACE);
-		return wires.stream().map(wire -> {
+		return wires.stream().filter(wire -> wire.getProviderWiring() != null).map(wire -> {
 			return wire.getProvider();
 		}).distinct().flatMap(provider -> {
 			IPluginModelBase model = PluginRegistry.findModel(provider);